Various apparatus and methods exist that employ an optical parametric oscillator (OPO), in some cases to generate terahertz radiation (having frequencies from about 0.3 THz to about 10 THz). Some examples are described in:    U.S. Pat. No. 5,017,806 entitled “Broadly tunable high repetition rate femtosecond optical parametric oscillator” issued May 21, 1991 to Edelstein et al;    U.S. Pat. No. 5,212,698 entitled “Dispersion compensation for ultrashort pulse generation in tuneable lasers” issued May 18, 1993 to Kafka et al;    U.S. Pat. No. 5,365,366 entitled “Synchronously pumped sub-picosecond optical parametric oscillator” issued Nov. 15, 1994 to Kafka et al;    U.S. Pat. No. 5,371,752 entitled “Optical parametric oscillation using KTA nonlinear crystals” issued Dec. 6, 1994 to Powers et al;    U.S. Pat. No. 5,377,043 entitled “Ti:sapphire-pumped high repetition rate femtosecond optical parametric oscillator” issued Dec. 27, 1994 to Pelouch et al;    U.S. Pat. No. 5,406,408 entitled “Intracavity-doubled tunable optical parametric oscillator” issued Apr. 11, 1995 to Ellingson et al;    U.S. Pat. No. 5,847,861 entitled “Synchronously pumped sub-picosecond optical parametric oscillator” issued Dec. 8, 1998 to Kafka et al;    U.S. Pat. No. 6,282,014 entitled “Cascade optical parametric oscillator for down-conversion” issued Aug. 28, 2001 to Long et al;    U.S. Pat. No. 7,272,158 entitled “Highly efficient waveguide pulsed THz electromagnetic radiation source and group-matched waveguide THz electromagnetic radiation source” issued Sep. 18, 2007 to Hayes et al;    U.S. Pat. No. 7,339,718 entitled “Generation of terahertz radiation in orientation-patterned semiconductors” issued Mar. 4, 2008 to Vodopyanov et al;    U.S. Pat. No. 7,349,609 entitled “Terahertz radiation generation and methods therefor” issued Mar. 25, 2008 to Vodopyanov et al;    U.S. Pat. No. 7,616,304 entitled “System and method for providing a tunable optical parametric oscillator laser system that provides dual frequency output for non-linear vibrational spectroscopy and microscopy” issued Nov. 10, 2009 to Gankkhanov et al;    U.S. Pat. No. 8,599,474 entitled “Alignment and optimization of a synchronously pumped optical parametric oscillator for nonlinear optical generation” issued Dec. 3, 2013 to Kozlov et al;    U.S. Pat. No. 8,599,475 entitled “Generation of terahertz radiation in orientation-patterned semiconductors” issued Dec. 3, 2013 to Kozlov et al;    U.S. Pat. No. 8,599,476 entitled “Generation of terahertz radiation in orientation-patterned semiconductors” issued Dec. 3, 2013 to Kozlov et al;    U.S. Pub. No. US 2008/0037595 entitled “System and method for providing a tunable optical parametric oscillator laser system that provides dual frequency output for non-linear vibrational spectroscopy and microscopy” published Feb. 14, 2008 in the names of Gankkhanov et al;    Pub. No. WO 2007/132229 entitled “Optical parametric oscillator” published Nov. 22, 2007 in the names of Ebrahim-Zadeh et al;    Pub. No. WO 2008/135257 entitled “Method and optical arrangement for generating a nonlinear optical signal on a material which is excited by an excitation field, and use of the method and of the optical arrangement” published Nov. 13, 2008 in the names of Rimke et al;    Kieu et al; “High-power picoseconds fiber source for coherent Raman spectroscopy”; Optics Letters Vol 34 p 2051 (2009);    Saar et al; “Intracavity wavelength modulation of an optical parametric oscillator for coherent Raman microscopy”; Optics Express vol 17 p 12532 (2009);    Schaar et al; “Intracavity terahertz-wave generation in a synchronously pumped optical parametric oscillator using quasi-phase-matched GaAs”; Optics Letters vol 32 p 1284 (2007);    Lee et al; “Generation of multicycle terahertz pulses via optical rectification in periodically inverted GaAs structures”; Applied Physics Letters vol 89 p 181104 (2006);    Imai et al; “A frequency-agile terahertz-wave parametric oscillator”; Optics Express Vol. 8, No. 13, p. 699 (2001);    Taniuchi et al; “Collinear phase-matched terahertz-wave generation in GaP crystal using a dual-wavelength optical parametric oscillator”; Journal of Applied Physics Vol. 95, No. 12, p. 7588 (2004);    Sowade et al; “Continuous-wave optical parametric terahertz source”; Optics Express Vol. 17, No. 25, p. 22303 (2009);    Sowade; “Continuous-wave terahertz light from optical parametric oscillators”; doctoral dissertation, University of Bonn (2010);    Petersen et al; “Enhanced terahertz source based on external cavity difference-frequency generation using monolithic single-frequency pulsed fiber lasers”; Optics Letters Vol. 35, No. 13, p. 2170 (2010);    Xu et al; “High Energy Terahertz Parametric Oscillator Based on Surface-Emitted Configuration”; Chinese Physics Letters Vol. 30, No. 2, p. 024212 (2013);    Li et al; “Investigation on terahertz parametric oscillators using GaP crystal with a noncollinear phase-matching scheme”; Journal of Modern Optics Vol. 62, No. 4, p. 302 (2015);    Vodopyanov et al; “Resonantly-enhanced THz-wave generation via multispectral mixing inside a ring-cavity optical parametric oscillator”; Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (Optical Society of America, 2009), CLEO paper CTuG1;    Kokabee et al; “Efficient, high-power, 16-GHz, picosecond optical parametric oscillator pumped by an 81-MHz fiber laser”; Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (Optical Society of America, 2010), CLEO paper CThP2;    Hurlbut et al; “THz-wave generation inside a high-finesse ring-cavity OPO pumped by a fiber laser”; Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (Optical Society of America, 2010), CLEO paper CWF3; and    Creeden et al., “Compact, high average power, fiber-pumped terahertz source for active real-time imaging of concealed objects”, Optics Express Vol. 15, No. 10, p. 6478 (2007).